Brake pad insulators are typically used on the backing plate of a disk brake. Brake pad insulators are designed to perform a damping function at various vibrations, frequencies and temperatures. The brake pad insulators thus help quiet vibrations and induced noise of the disk brake pad.
Brake pad insulators exploit various damping mechanisms to achieve the desired noise reduction. It is hypothesized that insulators attenuate noise by extensional damping, shear-deformation, isolation damping and frictional damping. Extensional damping may also be referred to as free layer damping and involves relative movement of a relatively soft layer of material that is over, on or otherwise connected with a relatively stiff layer of material. If the stiff layer bends the soft layer stretches but does not shear. Shear-deformation damping, also referred to as constrained damping, involves relative movement of a soft layer of material positioned between two stiff layers. The stiff layers act as constraints and shear occurs in the soft material but mostly only bending occurs in the stiffer material. Isolation damping is well understood to be the type of damping occurring when the insulator absorbs vibration pulses. For instance, vibrations between the brake pad and rotor may be damped by axial compression of the insulator.
Frictional damping is perhaps the least studied of the damping mechanisms occurring with a brake pad insulator. Frictional damping occurs between contacting surfaces that have relative slip. In the brake pad insulator example, the outermost layer of the brake pad insulator is contacted by a brake actuator such as a piston or a caliper finger. Relative slip may occur, resulting in surface friction damping.
There are many mathematical models that can be construed to represent frictional forces between contacting surfaces. The simplest and fundamental representation shown below, is the Coulomb friction model, which relates the total frictional (damping) force (Ff) between contacting bodies, to the net contact force (Fc), through the kinetic coefficient of friction μk. The direction of the frictional force depends upon the sign (sgn) of the relative velocity (v) between contacting surfaces. The value of sgn(v) is +1 or −1.Ff=−μk·Fc·sgn(v)The values of friction measured near zero relative velocities (i.e., at the transition from sticking contact to slipping contact,) lead to the static coefficient of friction, μ, which itself is usually a good indicator of the surface frictional damping that can occur between contacting and slipping surfaces. Therefore, frictional damping of a brake pad insulator can be quantified by determining the surface friction. Known methods of surface friction measurements are not suitable for brake pad insulator material. For example, the American Society for Testing and Materials (ASTM) Standard D 1894-01 is a standard method for determining the static and kinetic coefficients of friction of film and sheeting. The test method determines the coefficients of starting and sliding plastic film and sheeting when sliding over itself or other substances at specified test conditions. The procedure permits the use of a stationary sled with a moving plane, or a moving sled with a stationary plane. The sled and plane maybe referred to as a “sliding table” design. ASTM D 1894-01 standard method uses a pressure of less than one bar which makes the test unsuitable for measuring surface friction of brake pad insulators under realistic conditions, since the operating pressures in disk brakes are much higher, i.e., on the order of 10 times higher. Applying the method to a brake pad insulator would require a more rigid and frictionless sliding table. Also, it is cumbersome to test at temperatures other than ambient under ASTM D 1894-01, as the plane may be seated while the sled must be held at the ambient temperature. Finally, the ATSM D 1894-01 method is based on linear sliding instead of rotary, and is therefore not representative of a brake pad insulator application.
Known friction measurement test rigs for brake linings (i.e., brake pads) are relatively bulky and have high rigidity requirements due to the relatively large rotational displacement requirements and torque existing between a brake pad and a rotor. Insulators operate under conditions of much smaller relative motion (i.e., the movement between the insulator and the actuating brake piston or brake caliper finger) or load, making these friction measurements test rigs not quite suitable for brake pad insulator friction measurement.
It is desirable to know the surface friction characteristics of a damping material, such as brake pad insulator material, under specific conditions, such as likely in-use conditions including the ranges of temperatures, pressures and torques that the insulators will be subjected to, in order to evaluate the effect of surface friction on overall damping capability.